This research will involve studies in mammalian cells of the content and/or functional state in mitochondria of two important hemoproteins: cytochrome c and cytochrome oxidase; a neutral protease; and two enzymes of intermediary lipid metabolism: glycerol phosphate dehydrogenase and acetoacetyl CoA deacylase. Rat liver, kidney and cardiac muscle will be studied at three stages of development: active growth (10 weeks); maturity (10 months) and senescence (20 and 30 months). The effects of the hypophysectomized and thyroidectomized states, and of the restoration of growth hormone, thyroid hormone, and a second pituitary hormone to such animals, will be studied at the stage of early maturity. Such experiments will be constructed to assess the exact effects of each hormone on mitochondrial biogenesis and degradation. Possible changes of the functional state and numbers of mitochondria will be evaluated by following levels of mitochondrial DNA and protein, measurements of phosphorylation and respiration, and the capacity of mitochondria and ER, both in vivo and in vitro, to synthesize total mitochondrial protein, cytochrome c and cytochrome oxidase. Age-associated changes in mitochondria will be correlated with any changes in the character of the ER or isolated microsomes with any changes in the levels of neutral mitochondrial protease as well as with the protein and phospholipid content of mitochondrial membrane fractions. Any changes in the levels of the mitochondrial enzymes of intermediary lipid metabolism will be correlated with any changes observed in mitochondrial content of neutral protease and the serum and tissue contents of lipids.